Fewer than 50,000 people had successfully navigated the troubled federal health-care website and enrolled in private insurance plans as of last week, two people familiar with the matter said, citing internal government data.

The figure is a fraction of the Obama administration's target of 500,000 enrollees for October. The early tally for the HealthCare.gov site, which launched Oct. 1, worries health insurers that are counting on higher enrollment to make their plans profitable...

The administration thus far has said only that about 700,000 people had completed applications in all 50 states, using both the federal and state-run sites, in an Oct. 24 disclosure. An application is an early step for a consumer to determine pricing and eligibility for policies before enrolling.

Ms. Tavenner, who runs the Centers for Medicare & Medicaid Services, and her boss, HHS Secretary Kathleen Sebelius, both have told Congress that the initial numbers would be low.

So far, private health plans have received enrollment data for 40,000 to 50,000 users of the federal marketplace, the people familiar with the figures said.